1. Field of the Invention
This invention relates to a method for manufacturing a shadow mask for use in a color CRT.
2. Description of the Related Art
Generally a shadow mask type color CRT includes a shadow mask having a great number of apertures formed at a predetermined pitch. The shadow mask is arranged closely opposite a fluorescent screen with R (red), B (blue) and G (green) phosphor layers coated in a stripe pattern. The phosphor layers emit red, green and blue fluorescent lights by irradiation of electron beams from three corresponding electron guns. The apertures of the shadow mask allow the three electron beams to selectively pass therethrough so that the beams land exactly on the R, B and G phosphor layers. It is thus possible to reproduce a color image. That is, the shadow mask constitutes one important member having a color selection function. A variation in the shape and dimension of the apertures, positional shift of the apertures to the corresponding phosphor layers and displacement of a shadow mask-to-phosphor screen distance away from a predetermined position, if exceeding their allowable range, present a grave problem in terms of the characteristic of the color CRT, such as a decline in color purity for the color CRT.
The respective aperture of the shadow mask is generally so configured as to allow a predetermined amount of electron beam which is generally slantwise incident toward the shadow mask to pass therethrough. That is, each aperture H of the shadow mask is formed in thin metal mask sheet 1 such that, as shown in FIG. 1, it has opening 2 on the side of the phosphor screen and opening 3 on the side of electron guns. In this case, opening 2 (larger opening) has a greater size than that of opening 3 (smaller opening).
This type of shadow mask is manufactured in the following steps.
As shown in FIG. 2A, photosensitive resin layers, such as photosensitive material-containing resist films 4a and 4b are formed on both major surfaces of bandlike metal thin sheet 1 which is used for a shadow mask material. The resist film is exposed to light via a photomask pattern corresponding to the size of an array of apertures. Then openings 2' and 3' corresponding to larger and smaller openings 2 and 3 are formed respectively in resist films 4a and 4b by development. Sheet 1 is then subjected to etching using an etching solution suitable for etching a particular material forming the sheet. If metal sheet 1 consists principally of iron, it can be etched by an etching solution containing ferric chloride as a main constituent, to provide an aperture H as shown in FIG. 2C. Remaining resist films 4a, 4b are removed from band-like thin metal sheet 1 to provide a flat mask. The flat mask is shaped to provide a complete shadow mask.
In order to provide a predetermined aperture configuration as shown in FIG. 1, etching has to be controlled first for the "smaller opening" side most relevant to the accuracy of its minimum diameter and then for the "larger opening" side. In the process of etching, a better efficiency of exchange between a fresh etching solution and a "fatigued" etching solution is involved at the location of larger opening 2 and, as a result, the etching rate and etching amount are greater on the "larger opening" side than on the "smaller opening" side.
However, the following problem arises in the control of the etching step.
At the etching of the "larger opening" side, side etching progresses due to the greater etching rate and greater etching amount being involved. At that side etching, overhang portion 5 is formed at resist film 4a such that it extends toward a center axis of larger opening 2 as shown in FIG. 2C. That overhang portion 5 is often separated or destroyed, for example, with a pressure under which the etching solution is sprayed.
As a result, further etching progresses at the destroyed portion of resist film 4, causing a variation in the configuration and dimension of apertures obtained. This variation prominently occurs at an array of reduced-diameter apertures of shadow mask at a shorter pitch as in a high-definition color CRT. The thicker the thin-metal sheet, the greater the amount of etching and the greater the time taken from etching to be conducted. Thus the side etching is liable to progress, causing a variation in the configuration and dimension of apertures as set forth above.
In the step of exposing the resist to light, the light which passes through the pattern mask is diffused in resist film 4 on thin metal sheet 1. At this time, light is passed through the pattern mask such that it exposes the resist film weakly at its thicker portion and strongly at its thinner portion. The resist film, if not uniform, involves a decline in dimensional accuracy of the openings involved. It is important that resist film 4 be formed, as a uniform thickness film, under a constant exposure condition.
As an ordinary resist coating method use is made of a flow coat method as shown in FIG. 3 and a dip coat method as shown in FIG. 4.
In the flow coat method as shown in FIG. 3, bandlike thin metal sheet 1 is vertically erected with one side edge thereof down and sequentially fed while being coated at each surface with a resist solution (resist material 6) in a down-flow fashion. The metal sheet enters drying furnace 7 where it is dried to obtain a resist film. The resist film obtained in thinner at the upper side portion than at the lower side portion and, that is, a film thickness difference occurs across the width of the band-like metal sheet due to a gravity action. A uniform film thickness cannot be obtained even if, in order to cancel such a film thickness difference, use is made of a drying furnace having such a temperature distribution as to allow, for example, the upper portion of the metal sheet to be dried at a faster rate than the lower portion thereof or even if the conveying speed of the thin metal sheet, viscosity of a resist film, and so on vary in various conditions.
In the dip coat method as shown in FIG. 4, bandlike thin metal sheet 1, while being conveyed in the longitudinal direction, is dipped into resist tank 8 holding resist solution 6, and passes, while being lifted off in a vertical direction, through drying furnace 9 where it is dried. The resist film thus obtained involves a film thickness difference in the longitudinal direction due to the coated resist solution flowed by gravity down along each surface until it is fixed to the each surface of the film. It is not possible to obtain a uniform thickness resist film even if conditions such as the viscosity of the resist solution, lift-off speed of the metal film, drying temperature distribution in the drying furnace, and amount of air blown into the drying furnace are varied so as to eliminate a film thickness difference.
As evident from the above, the coated metal sheet is unavoidably adversely affected by the gravity, failing to obtain a uniform thickness resist film.
If thin metal sheet 1 is thickened, side etching is liable to progress due to a longer etching time involved so that the aforementioned resist film is separated or destroyed at its overhang portion 5. To combat this problem, the resist film is thickened to improve the mechanical strength of resist film 4. If, in the conventional method, the resist film is re-set to a greater thickness, however, it is necessary to largely vary the viscosity of the resist as well as the coating and drying conditions. Even if these steps are done in the aforementioned method, it is not possible to obtain a uniform resist film due to a gravity involved. It is thus not possible to simply obtain resist films of a uniform thickness on a quantity production line.
A better resolution of resist film 4 is desired so as to obtain apertures of better dimensional accuracy in the resist film. The resolution depends upon the characteristic of the resist material and thickness of the resist film. The thinner the resist film, the higher the resolution. In the etching step, unless overhang portion 5 is separated or destroyed, the resist film on the side of smaller opening 3, which determines the dimension of apertures in the shadow mask in particular, is better be made as thin as possible in comparison with that on the side of larger aperture 2. Japanese Patent Disclosure (KOKAI) No. 60-70185 proposes, for example, a two-stage etching method whereby, even if the resist film is made thinner, no overhang portion is broken at the location of the apertures due to the etching time shorter on the side of the smaller openings. The use of this method enables the resist film to be made thinner without degrading the quality of the shadow mask.
Thus various means have been proposed to manufacture a high-definition shadow mask, but it has been very difficult to uniformly coat a resist film which is responsible for the dimensional accuracy of the apertures obtained.